Binary digital data is typically recorded on mass storage media as a pattern of transitions in a storage medium. For example, data on magnetic storage media is represented by changes of magnetic polarity, and data on optical storage media may be represented by changes in reflectivity or transmissivity. The transition patterns correspond to digital data which have been encoded to facilitate recording. When a magnetic memory is read using an inductive read head, or an optical memory is read using an optical scanner, an analog signal is generated with relative positive and negative peaks or large and small signals corresponding to the transition pattern. The analog signal, which may be distorted by system noise and other influences, which may be non-linear, is then demodulated to extract the original transition pattern as faithfully as possible and interpret it as a series of binary encoded bits. The binary encoded bits must then be decoded to reproduce the original digital data.
Signal demodulation may become increasingly difficult as the density of data recorded on the disks is increased. With higher recording densities, the medium, or disk, space allotted for the recording of a transition signal, that is, a transition cell, is, in effect, reduced. The signals read from the transition cells relative to the background noise tend to be smaller as a result and they can be more readily misinterpreted.
The signals may be misinterpreted because of system noise, which can distort the signals read from the "small" transition cells, or because of interference from surrounding transition cells, which can cause signal peaks to shift within a transition cell or even to adjacent transition cells. The misinterpretation of the transition signals results in errors in the binary encoded bits which, after decoding, results in errors in the digital data.
The extent of the resulting digital data errors, due to misinterpretation of the signal because of distortion from noise or peak shifts, is categorized as a soft bit error rate. Soft errors must be corrected by the system's error correction process before the data can be used in data processing. Thus the error rate that the system error correction code ("ECC") can correct implicitly becomes a limitation on the recoding density. If recording densities are to increase over current densities, either the ratio of signal-to-system noise, including peak shifts, must be improved or the signal processing which is used to generate and recover recorded signals must be improved. One method for such improvement is to more faithfully interpret recorded signals during the demodulation process.
Various demodulation systems, which are aimed at faithful signal interpretation, are currently in use. Some less complicated systems employ amplitude detectors and peak detectors which detect the presence of signals with amplitudes, or peaks, within certain ranges and interprets them as binary encoded bits. Other, more sophisticated, systems employ various forms of signal processing to filter and enhance the analog signal before the amplitudes or peaks are detected. These techniques work well for current recording densities. However, as recording densities increase and as the likelihood of distortion by system noise and peak shift by adjacent-cell interference also increases, the demodulation techniques must become more sophisticated to faithfully interpret the binary encoded bits.